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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration risks (Details)
3 Months Ended
Mar. 31, 2020
USD ($)
customer
Mar. 31, 2019
USD ($)
customer
Concentration risks    
Aggregate dollar amount of net sales to significant customers $ 11,202,217 $ 7,932,474
Customer Concentration Risk    
Concentration risks    
Number of significant customers | customer 3 3
Aggregate dollar amount of net sales to significant customers $ 5,400,000 $ 3,900,000
Percentage of net sales to significant customers 48.20% 48.60%
Supplier Concentration Risk    
Concentration risks    
Percentage of net sales to significant customers 80.30% 76.10%